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          LV02-02-shell-05-重定向
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              <table>    <tr>        <td align="center" width=150px>Windows</td>        <td align="left">windows11</td>    </tr>    <tr>        <td align="center">Ubuntu</td>        <td align="left">Ubuntu16.04的64位版本</td>      </tr>    <tr>        <td align="center">VMware® Workstation 16 Pro</td>        <td align="left">16.2.3 build-19376536</td>      </tr>    <tr>        <td align="center">SecureCRT</td>        <td align="left">Version 8.7.2 (x64 build 2214)   -   正式版-2020年5月14日</td>      </tr>    <tr>        <td align="center">开发板</td>        <td align="left">正点原子 i.MX6ULL Linux阿尔法开发板</td>      </tr>    <tr>        <td align="center">uboot</td>        <td align="left">NXP官方提供的uboot，NXP提供的版本为uboot-imx-rel_imx_4.1.15_2.1.0_ga(使用的uboot版本为U-Boot 2016.03)</td>      </tr>    <tr>        <td align="center">linux内核</td>        <td align="left">linux-4.15(NXP官方提供)</td>      </tr>    <tr>        <td align="center">STM32开发板</td>        <td align="left">正点原子战舰V3(STM32F103ZET6)</td>      </tr></table>
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<p>Unix  命令默认从标准输入设备( stdin )获取输入，将结果输出到标准输出设备( stdout )显示。一般情况下，标准输入设备就是键盘，标准输出设备就是终端，即显示器。</p>
<h1 id="1-输出重定向"><a href="#1-输出重定向" class="headerlink" title="1.输出重定向"></a><font size=3>1.输出重定向</font></h1><h2 id="1-1输出重定向的概念"><a href="#1-1输出重定向的概念" class="headerlink" title="1.1输出重定向的概念"></a><font size=3>1.1输出重定向的概念</font></h2><p>命令的输出不仅可以是显示器，还可以输出到某个文件，这就叫做输出重定向。</p>
<table>
    <tr>
        <td align="center" width=150px>命令</td>
        <td align="center">说明</td>
    </tr>
    <tr>
        <td align="center" width=150px>command > file</td>
        <td align="left">将输出到显示器的内容重定向到 file文件。<br>【注意】file文件中原有的内容会被清空</td>
    </tr>
    <tr>
        <td align="center" width=150px>command >> file</td>
        <td align="left">将输出到显示器的内容以追加的方式重定向到 file文件。<br>【注意】file文件中原有的内容会保留，新的内容追加在结尾。</td>
    </tr>
</table>

<p>【注意】 &gt; 和 &gt;&gt; 两个符号都是起到重定向的作用，但是，前者会清空原有文件，但是后者则为追加。</p>
<h2 id="1-2使用实例"><a href="#1-2使用实例" class="headerlink" title="1.2使用实例"></a><font size=3>1.2使用实例</font></h2><figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta prompt_"># </span><span class="language-bash">命令行中执行</span></span><br><span class="line">who &gt; file_user   # 将命令完整的输出重定向在用户文件中(file_user)</span><br><span class="line">date &gt;&gt; file_user # 将日期追加到file_user文件结尾</span><br><span class="line">ls ~ &gt; file_user  # 清空file_user，将家目录中所有文件的名称重定向到file_user文件</span><br></pre></td></tr></table></figure>

<p>运行结果如下图：</p>
<img data-src="https://fanhua-picture.oss-cn-hangzhou.aliyuncs.com/01%E5%B5%8C%E5%85%A5%E5%BC%8F%E5%BC%80%E5%8F%91/01HQ%E8%AF%BE%E7%A8%8B%E4%BD%93%E7%B3%BB/LV02-%E7%BC%96%E8%AF%91%E5%9F%BA%E7%A1%80/LV02-02-shell-05-%E9%87%8D%E5%AE%9A%E5%90%91/img/image-20220216153538484.png" style="zoom:50%;" />

<h1 id="2-输入重定向"><a href="#2-输入重定向" class="headerlink" title="2.输入重定向"></a><font size=3>2.输入重定向</font></h1><h2 id="2-1输入重定向的概念"><a href="#2-1输入重定向的概念" class="headerlink" title="2.1输入重定向的概念"></a><font size=3>2.1输入重定向的概念</font></h2><p> Unix  命令也可以从文件获取输入，而不是默认的键盘</p>
<table>
    <tr>
        <td align="center" width=150px>命令</td>
        <td align="left">说明</td>
    </tr>
    <tr>
        <td align="center" width=150px>command < file </td>
        <td align="left">本来需要从键盘获取输入的命令会转移到文件file中读取内容。</td>
    </tr>
</table>


<h2 id="2-2使用实例"><a href="#2-2使用实例" class="headerlink" title="2.2使用实例"></a><font size=3>2.2使用实例</font></h2><figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta prompt_"># </span><span class="language-bash">命令行中执行</span></span><br><span class="line">wc -l file_user</span><br><span class="line">wc -l &lt; file_user </span><br></pre></td></tr></table></figure>

<p>运行结果如下图：</p>
<img data-src="https://fanhua-picture.oss-cn-hangzhou.aliyuncs.com/01%E5%B5%8C%E5%85%A5%E5%BC%8F%E5%BC%80%E5%8F%91/01HQ%E8%AF%BE%E7%A8%8B%E4%BD%93%E7%B3%BB/LV02-%E7%BC%96%E8%AF%91%E5%9F%BA%E7%A1%80/LV02-02-shell-05-%E9%87%8D%E5%AE%9A%E5%90%91/img/image-20220216154305040.png" style="zoom:67%;" />

<p>第一个命令没有进行重定向，会输出文件名；第二个命令却不会，因为第二个命令仅仅知道从标准输入读取内容。</p>
<h1 id="3-重定向的深入理解"><a href="#3-重定向的深入理解" class="headerlink" title="3.重定向的深入理解"></a><font size=3>3.重定向的深入理解</font></h1><h2 id="3-1文件描述符"><a href="#3-1文件描述符" class="headerlink" title="3.1文件描述符"></a><font size=3>3.1文件描述符</font></h2><p>文件描述符， File descriptor ，简称 fd ，当应用程序请求内核打开&#x2F;新建一个文件时，内核会返回一个文件描述符用于对应这个打开&#x2F;新建的文件，其 fd <strong>本质上就是一个非负整数</strong>。实际上，它是一个索引值，指向内核为每一个进程所维护的该进程打开文件的记录表。当程序打开一个现有文件或者创建一个新文件时，内核向进程返回一个文件描述符。在程序设计中，一些涉及底层的程序编写往往会围绕着文件描述符展开。但是文件描述符这一概念往往只适用于 UNIX 、 Linux 这样的操作系统。</p>
<p><strong>文件描述符  0  通常是标准输入（ STDIN ）， 1  是标准输出（ STDOUT ）， 2  是标准错误输出（ STDERR ）</strong>。</p>
<p>一般情况下，每个  Unix&#x2F;Linux  命令运行时都会打开三个文件</p>
<table>
    <tr>
        <td align="left" width=200px>标准输入文件(stdin)</td>
        <td align="left">stdin的文件描述符为0，Unix程序默认从stdin读取数据。</td>
    </tr>
    <tr>
        <td align="left" width=200px>标准输出文件(stdout)</td>
        <td align="left">stdout 的文件描述符为1，Unix程序默认向stdout输出数据。</td>
    </tr>
    <tr>
        <td align="left" width=200px>标准错误文件(stderr)</td>
        <td align="left">stderr的文件描述符为2，Unix程序会向stderr流中写入错误信息。</td>
    </tr>
</table>


<h2 id="3-2重定向命令"><a href="#3-2重定向命令" class="headerlink" title="3.2重定向命令"></a><font size=3>3.2重定向命令</font></h2><p>默认情况下， command &gt; file  将  stdout  重定向到  file ， command &lt; file  将 stdin  重定向到  file 。所有可用重定向命令如下：</p>
<table>
    <tr>
        <td align="center">输出重定向命令</td>
        <td align="left">说明</td>
    </tr>
    <tr>
        <td align="center">command &gt; file</td>
        <td align="left">将输出到显示器的内容重定向到 file文件。<br>【注意】file文件中原有的内容会被清空</td>
    </tr>
    <tr>
        <td align="center">command &gt;&gt; file</td>
        <td align="left">将输出到显示器的内容以追加的方式重定向到 file文件。<br>【注意】file文件中原有的内容会保留，新的内容追加在结尾。</td>
    </tr>
    <tr>
        <td align="center">n &gt; file</td>
        <td align="left">将文件描述符为 n 的内容重定向到 file文件。<br>【注意】file文件中原有的内容会被清空。</td>
    </tr>
    <tr>
        <td align="center">n &gt;&gt; file</td>
        <td align="left">将文件描述符为 n 的内容以追加的方式重定向到 file文件。<br>【注意】file文件中原有的内容会保留，新的内容追加在结尾。</td>
    </tr>
    <tr>
        <td align="center">n &gt;& file</td>
        <td align="left">将输出文件 file 和 n 合并。</td>
    </tr>
        <tr>
        <td align="center">输入重定向命令</td>
        <td align="left">说明</td>
    </tr>
    <tr>
        <td align="center">command &lt; file </td>
        <td align="left">本来需要从键盘获取输入的命令会转移到文件file中读取内容。</td>
    </tr>
    <tr>
        <td align="center">n &lt;& file</td>
        <td align="left">将输入文件 file 和 n 合并。</td>
    </tr>
    <tr>
        <td align="center"> &lt;&lt; tag </td>
        <td align="left">将开始标记 tag 和结束标记 tag 之间的内容作为输入。</td>
    </tr>
</table>


<h2 id="3-3-stderr-重定向到-file"><a href="#3-3-stderr-重定向到-file" class="headerlink" title="3.3 stderr  重定向到  file "></a><font size=3>3.3 stderr  重定向到  file </font></h2><figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">command 2 &gt; file # 清空原文件再写入</span><br><span class="line">command 2 &gt; file # 追加到原文件后面</span><br></pre></td></tr></table></figure>

<h2 id="3-4-stdout-和-stderr-合并后重定向到-file"><a href="#3-4-stdout-和-stderr-合并后重定向到-file" class="headerlink" title="3.4 stdout  和  stderr  合并后重定向到  file "></a><font size=3>3.4 stdout  和  stderr  合并后重定向到  file </font></h2><figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">command &gt; file 2&gt;&amp;1</span><br><span class="line">command &gt;&gt; file 2&gt;&amp;1</span><br></pre></td></tr></table></figure>

<p>【注意】这里的  2  和  &gt; 之间不可以有空格， 2&gt; 是一体的时候才表示错误输出。</p>
<h2 id="3-5-stdin-和-stdout-都重定向"><a href="#3-5-stdin-和-stdout-都重定向" class="headerlink" title="3.5 stdin  和  stdout  都重定向"></a><font size=3>3.5 stdin  和  stdout  都重定向</font></h2><figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">command &lt; file1 &gt;file2</span><br></pre></td></tr></table></figure>

<p> command  命令将  stdin  重定向到  file1 ，将  stdout  重定向到  file2 。</p>
<h2 id="3-6-Here-Document"><a href="#3-6-Here-Document" class="headerlink" title="3.6 Here Document "></a><font size=3>3.6 Here Document </font></h2><p> Here Document  是  Shell  中的一种特殊的重定向方式，它的基本的形式如下:</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta prompt_"># </span><span class="language-bash">将两个 delimiter 之间的内容(document) 作为输入传递给 <span class="built_in">command</span></span></span><br><span class="line">command &lt;&lt; delimiter</span><br><span class="line">    document</span><br><span class="line">delimiter</span><br></pre></td></tr></table></figure>

<p>【注意】</p>
<p>（1）结尾的 delimiter  一定要顶格写，前面不能有任何字符，后面也不能有任何字符，包括 Space 和  Tab  缩进。</p>
<p>（2）开始的 delimiter 前后的空格会被忽略掉。</p>
<p>例如：</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta prompt_">#</span><span class="language-bash">!/bin/bash</span></span><br><span class="line">cat &lt;&lt; tag</span><br><span class="line">Hello,World!</span><br><span class="line">qidaink</span><br><span class="line">tag</span><br></pre></td></tr></table></figure>

<p>输出结果：</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">Hello,World!</span><br><span class="line">qidaink</span><br></pre></td></tr></table></figure>

<h1 id="4-dev-null-文件"><a href="#4-dev-null-文件" class="headerlink" title="4. &#x2F;dev&#x2F;null  文件"></a><font size=3>4. &#x2F;dev&#x2F;null  文件</font></h1><p>如果希望执行某个命令，但又不希望在屏幕上显示输出结果，那么可以将输出重定向到  &#x2F;dev&#x2F;null 。 &#x2F;dev&#x2F;null  是一个特殊的文件，写入到它的内容都会被丢弃；如果尝试从该文件读取内容，那么什么也读不到。但是  &#x2F;dev&#x2F;null  文件非常有用，将命令的输出重定向到它，会起到”禁止输出”的效果。</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">command &gt; /dev/null 2&gt;&amp;1 # 屏蔽 stdout 和 stderr</span><br></pre></td></tr></table></figure>
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